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Working of oscillator-with Quartz crystal

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Member level 4
Nov 6, 2005
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working of oscillator

Hi all,

I have attached a colpitt oscillator, which has quartz crystal in the feedback. When i tested the circuit without the crystal it is operating at the required frequency, but when I insert the crystal in the feedback loop it is oscillating.

I would like to know, what might be the possible resons behind this, please help me on this.


working of oscillator

did you mean when you puts the crystal it does not oscillating

working of a oscillator

yes, when I put the crystal in the feedback path it is not oscillating.


working of an oscillator

Can anyone tell me the function of inductor L10?

working of oscillator


It is used for pulling the resonant frequency of the oscillator.


Are you sure you are not overdriving the xtal?

crystals above 30mhz mostly run in overtone mode

keep this in mind

134mhz direct frequency is hard to achieve from a thin sliver of quartz

maybe reduce the xtal to a sub harmonic
and run in overtone mode is best and most stable

appart form this the transistor needs a load resistor added

or as pointed out youll overdrive it

? what supply voltage are you using ??? 5v dc ??

did you measure dc bias and check its right ???

should be ~ .707v base-emiter

r12 emiter side {between the choke coil and r12
should also have a decoupling cap added to gnd

or the resistor may act as a spring instead and dampen the oscilation}
adding an electrolytic 1uf @10v or 10nf ceramic will help here

and why does L10 have a 0R resistance ????

0R = 0 reactance also


Thanks for the reply.

I am using overtone crystal. I am using a 5th overtone crystal.

For the load resistor I am using 100k.

Supply voltage is 3V.

I removed the inductor in series with the crystal as I dont need to pull the crystal. Hence i named it 0R. I will try using capacitor in parallel with the emitter resistor.

Meanwhile, I didnt undertsand the overdrive concept, can anyone expalin in detail about this concept.


At this frequency you will need about 200-300nH across the crystal terminals to resonate out the 5-7pF of holder capacitance for the crystal to operate as it should. Just make sure the inductor has a SRF higher than the frequency you want to use.

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